1. Field of the Invention
The present invention relates to a travelling wave amplifier (TWA), in particular, a TWA for driving an optical modulator.
2. Background Arts
Recent optical communication system implements a transmitter optical module having an optical source, typically a semiconductor laser diode (LD), to emit continuous wave (CW) light combined with an optical modulator such as an electro-absorption modulator (EAM) and/or a Mach-Zehnder modulator (MZM) to modulate the CW light. The transmitter optical module is requested to be operable in a speed exceeding 25 Gbps or sometimes reaching 40 Gbps, where rising and falling times shorter than 10 ps as a large signal performance and a good S-parameter as a small signal performance are inevitable for achieving superior transmission performance. Accordingly, the transmitter optical module applicable to such high speed system usually installs a diver for driving the modulator configured with a TWA.
In order to enhance the capacity of the communication system, recent transmitter optical modules may have an arrangement of the multi-level modulation, for instance, 4-levels pulse-amplitude modulation (4-PAM) and/or 16-levels quadrature-amplitude modulation (16-QAM). The multi-level modulation inevitably requires a gain bandwidth substantially flat to a modulation frequency. For instance, a PAM-n algorithm, where n is an integer greater than unity (1), with a speed of 32 Gbps requires the flat gain bandwidth of 30 GHz, and 50 GHz bandwidth for a speed of 56 Gbps.